Commerce Bancshares Corp., the holding company for Commerce Bank & Trust Co., announced Monday that it has completed its acquisition of the Boston-based Mercantile Capital Corp. and its subsidiary, Mercantile Bank & Trust Co.

The $26.5 million all-cash transaction was announced earlier this year, the parent of Worcester-based Commerce Bank said.

Mercantile Bank’s three banking offices, located in the Brighton, Fenway, and South End neighborhoods of Boston, opened Monday as Commerce Bank offices, the company said.

The combined organization has assets of $1.6 billion and 16 retail banking offices serving people and businesses across Central and Eastern Massachusetts.

The deal comes at a time when the ranks of tiny banks have been thinning across the country